This invention relates to the production of a lightweight, high strength and stiffness composite structure that provides superior performance for a wide variety of applications, especially in recreational products such as tennis racket frames, skis and hockey sticks, and in body protective equipment such as football helmets, motorcycle helmets, shin guards and shoulder pads. The invention also provides a method of making such a structure.
The outstanding characteristics and benefits that the structure produced by this invention offers over products of the prior art are especially described in this specification with reference to tennis rackets as an example. However, as will be readily apparent to those skilled in the art, the product and the method of this invention provide superior characteristics for many applications where a material is desired that possesses low weight combined with high strength, stiffness, torsional resistance, excellent stability and long endurance.
The earliest tennis rackets utilized frames of solid wood, but these were superseded by laminated wood construction, such as selected ash, maple and birch laminates, sometimes with built-in, steel-like fibers. While the laminated wood construction offers improvements over solid wood, it suffers disadvantages such as relatively low tensile and compressive strength, poor life and lack of uniformity. In addition, it is subject to warpage and scuffing, tends to lose color, loses stiffness and strength properties, has a relatively short fatigue life, offers high wind resistance in cross sections large enough to provide the needed strength, and is difficult to manufacture to uniform weight, balance, density and mechanical properties.
Tennis rackets with metal frames, such as tubular, round, channel, I-beam extrusions and other configurations, have been constructed from alloy steel, magnesium, and especially aluminum. With the metal frames, there is a limit on weight to achieve the desired performance, and shock and vibration are transmitted to the player, especially on mis-hit shots, which contributes to player fatigue and to the "tennis elbow" injury. Objectionable sound transmission (sometimes rings) is another drawback of metallic frames. The metal frames also cause a trampoline effect. In addition, the grommets or eyelets used to guide the tennis string tend to crack prematurely.
Composite laminated fiber glass reinforced plastic tennis rackets have been produced by techniques such as injection molding, transfer molding, and compression molding. U.S. Pat. No. 3,483,055 to R. W. Eshbaugh describes a fiber glass construction for tennis racket frames in which the frames are made of alternating layers of resin-impregnated glass fibers and of absorbent paper. Such fiber glass tennis rackets are generally regarded as being too flexible and the desired rigidity cannot be obtained.
It is technically feasible to produce a similar composite structure using resin-impregnated graphite fibers in place of the glass fibers but such a structure still does not have the desired deflection characteristics.
A sandwich construction in tennis rackets, made by adhering glass fiber layers over both faces of a wooden racket frame results in improved service life, especially with regard to fatigue characteristics, but the tennis strings tend to loosen, dynamic response is lost, and there is a limitation on weight reduction.
An improved sandwich construction is obtained by adhering graphite fiber layers over a wooden frame, but such a construction still has the above-mentioned limitations of a wood core.
Sandwich constructions with cores other than wood also have been developed or investigated. These include a foamed plastic core with aluminum facing, and low density, preformed rigid plastic cores with aluminum, fiber glass or graphite fiber facings. These constructions suffer a number of disadvantages including creep and relaxation of tension on the strings. In the cases of metal facings, the mismatch in co-efficient of thermal expansion between the facing and core causes problems in fabrication and during use because of residual stresses.
Still another sandwich construction is described in U.S. Pat. No. 3,640,533 to T. B. Davis. This construction is comprised of a reinforcement mesh of metal covered on opposite sides by fiber glass reinforced resin, which is attached to the wood core of the racket. The metal mesh has a degrading effect on the composite due to the differences in elastic properties between the metal and the wood, and the system cannot be made as light as the composite structure of the instant invention and still achieve the same level of stiffness and resistance to fatigue.
Still another composite structure, useful in skis, is disclosed in U.S. Pat. No. 3,493,240 to H. R. Jenks. This structure comprises upper and lower skins of resin-impregnated fiber glass sheets bonded to longitudinally extended channel members, which provide hollow cores. The construction is difficult to process, complex, and susceptible to a great number of variables in production. Further, leaks in the air bags (hollow cores) may not be known until the part is constructed. Further, the fiber glass facing does not provide the stiffness-to-strength ratio provided by this invention.
In coassigned application Ser. No. 442,204, filed Feb. 13, 1974 by Andrew M. Cecka and Pol Dano, now abandoned, there is described a structural member comprising an expanded foamed plastic core integrally bonded to a shell comprising a plurality of layers of resin-coated unidirectionally oriented graphite fibers at least one of the layers having fibers oriented in a direction different from the direction of orientation in at least one other layer, the shell completely encasing the core at any transverse cross section of the structural member. The structural member is prepared by arranging within a mold cavity an outer shell made of a plurality of layers of unidirectionally oriented graphite fibers and a core comprising a foamable resin composition, sealing the mold and activating the foamable resin composition to cause expansion and generate pressure within the mold cavity and thereby provide intimate bonding of the core to the shell.
Structural members, and specifically tennis rackets, made in accordance with the aforementioned invention have been made and sold and have been well received by both amateur and professional players because of their unique combination of low weight, high strength, stiffness, torsional resistance, excellent stability and long endurance.
It has been found, however, that the core compositions heretofore used do not provide optimum processing characteristics and do not provide optimum properties in the final product.
It is desired to obtain substantial autogenous pressure in the mold cavity during the fabrication of the tennis rackets but such pressure must be obtained without the formation of excessively large gas voids in the core in order to preserve the strength of the structure.
In a preferred embodiment, described in concurrently filed and coassigned application Ser. No. 703,136, now U.S. Pat. No. 4,070,021 of Andrew M. Cecka, Pol Dano, and Paul G. Pawling, the core is inserted into a seamless sleeve made of a thin, flexible cellulosic film. The core is inserted into the sleeve endwise by air current and the core composition must be of sufficient stiffness and integrity to be capable of such insertion.
In addition, since different players prefer tennis rackets in different weights, it is desired to provide core compositions of different densities which have substantially the same pressure generating properties during fabrication and substantially the same structural properties in the final product.